如何从firebase检索特定的数据列表？

Question

有没有办法从孩子的孩子那里得到结果。 我正在寻找一个问题的多个答案。！

"questions" : {
    "Q1(Random ID )" : {
      "description" : "Deail of Question",
      "idQuestion" : "Q1",
      "time" : "17 Mar 2017 16:18:12",
      "title" : "Title Of Question",
      "user_id" : "bXCeW6jfidbHuMCCCMkDGWcGZRS2"
    },
{
  "Answer" : {
    "81d19e85-8c6c-4824-9fb1-61a7cd316e32" : {
      "dateTime" : "17 Mar 2017 16:22:17",
      "Answer" : "Answer",
      "ProfessorId" : "da587c89-f001-43f2-984d-e6dbac491acc",
      "questionId" : "Q1"
    },
    "e2ec0556-4420-4d2f-b346-0b5cebeb1ebe" : {
      "dateTime" : "17 Mar 2017 16:55:27",
      "Answer" : "Answer 2 on Question 1",
      "answerId" : "0d20d6df-907d-4133-be53-ca6acf6c2ad1",
      "questionId" : "Q1"
    }
  },

随机生成的节点。 ！ 想做这样的事情.!

mdatabaseReference.child("Answer").child(* Here Im Using "*" Like we used to do in SQl to get all data I knw its not a SQL but its just for conecpt).equalTo(QID).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

                if (Answer_List.size() > 0)
                    Answer_List.clear();


                for (DataSnapshot postsnapshot : dataSnapshot.getChildren()) {

而不是使用postSnapshot.getKey().equals("QID")

    if(postSnapshot.getKey().equals("QID"))  
 Answertext = postSnapshot.getValue().toString();

它会使性能变慢我正在寻找更快的东西。！

寻找这样的东西

 mdatabaseReference.child("Answer").child("*").getKey().equals(QID).addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {

Answer 1

您正在寻找Firebase 数据库查询。

mdatabaseReference
  .child("Answer")
  .orderByChild("questionId")
  .equalTo("Q1")
  .addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
      for (DataSnapshot answerSnapshot: dataSnapshot.getChildren()) {
        System.out.println(answerSnapshot.child("Answer").getValue());
      }
    }
    ...

然而，虽然这会奏效，但我强烈建议您更改数据模型以使此类查询更具可扩展性。 由于答案属于问题，您应该对该层次结构进行建模。 虽然您不应该混合问题和答案，但您应该考虑在问题 ID 下对答案进行建模：

"questions" : {
    "Q1(Random ID )" : {
      "description" : "Deail of Question",
      "idQuestion" : "Q1",
      "time" : "17 Mar 2017 16:18:12",
      "title" : "Title Of Question",
      "user_id" : "bXCeW6jfidbHuMCCCMkDGWcGZRS2"
    },
}
"Answer" : {
  "Q1": {
    "81d19e85-8c6c-4824-9fb1-61a7cd316e32" : {
      "dateTime" : "17 Mar 2017 16:22:17",
      "Answer" : "Answer",
      "ProfessorId" : "da587c89-f001-43f2-984d-e6dbac491acc",
    },
    "e2ec0556-4420-4d2f-b346-0b5cebeb1ebe" : {
      "dateTime" : "17 Mar 2017 16:55:27",
      "Answer" : "Answer 2 on Question 1",
      "answerId" : "0d20d6df-907d-4133-be53-ca6acf6c2ad1",
    }
  }
}

使用此结构，您可以通过直接查找而不是查询（在每个不断增长的列表中）获得 Q1 的答案：

mdatabaseReference
  .child("Answer")
  .child("Q1")
  .addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
      for (DataSnapshot answerSnapshot: dataSnapshot.getChildren()) {
        System.out.println(answerSnapshot.child("Answer").getValue());
      }
    }
    ...